Cancer chemotherapy often induces cardiotoxicity, which can have a significant impact on the overall prognosis and survival of cancer patients. Current guidelines to screen for cancer therapy-related cardiotoxicity are primarily based on serial assessment of left ventricular ejection fraction (EF), which is not a sensitive index of cardiotoxicity and may only decline at a time point that is too late to reerse the process. In addition to cardiac function, the microvasculature plays a critical rol in cardiotoxicity. There is a close bidirectional coupling of regional myocardial mechanics and microvascular perfusion. Many of the newer chemotherapy agents can directly cause microvascular injury, which may precede any EF drop. Due to an increasing aging population and rapid introduction of new therapy agents, more patients and cancer survivors are expected to suffer from cardiotoxicity. Therefore, there is an urgent need to develop novel non-invasive imaging techniques that might allow early detection of microvascular injury of patients with cardiotoxicity prior to a drop in EF. With this urgent clinical need, we propose to quantify Intramyocardial blood volume (IMBV) as a novel measurement of microcirculation function. 99mTc-labeled red blood cell (RBC) is a clinically available blood pool tracer for EF measurement and RBC imaging using Single Photon Emission Computed Tomography (SPECT) is a natural approach to estimate IMBV as the tracer stays in the intravascular circulation. However, accurate quantification of IMBV using SPECT is challenging, because 99mTc-RBC has ~5-6 fold higher activity in the blood pool than in myocardium, the spill-over counts from blood pool to the myocardium mainly due to poor resolution and respiratory/cardiac motion can cause substantial IMBV overestimation. We have developed various novel quantitative low-dose SPECT/CT methods including CT-based partial volume correction and motion corrections, and have demonstrated the feasibility of quantifying IMBV using SPECT/CT in large animal studies. We hypothesize that accurate measurement of IMBV can provide an early index of disruption of the microcirculation and vascular reserve and improve detection of cancer therapy induced cardiotoxicity. In this proposal, we will optimize, validate, and translate this low-dose (<2 mSv) quantitative SPECT/CT imaging approach into large animal and human studies. We will pursue the study through four Specific Aims. In Aim 1, we will optimize the low-dose SPECT/CT imaging approaches. In Aim 2, we will optimize the low-dose contrast CT data acquisition protocols. In Aim 3, we will quantify and validate the serial changes of IMBV in an established large animal model. In Aim 4, we will establish the feasibility of this SPECT/CT imaging approach in patient studies. This project is a stepping-stone to translate this imaging method to large clinical trials and clinical practice.